1. Field of the invention
The present invention relates to a dark level ring circuit for use with a television receiver.
2. Description of the Prior Art
An example of a conventional dark level restoring circuit (U.S. Pat. No. 4,631,589) will be described below with reference to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, a video signal applied to an input terminal 1 from a video signal source (not shown) is supplied to a pedestal clamp circuit 2, in which pedestal level of the input video signal is clamped to reference pedestal level of a reference voltage source 3. A video signal from the pedestal clamp circuit 2 is supplied to a dark detector/dark expander (dark level detector/dark level expander) 5 and a synthesizer (adder) 4.
The dark detector/dark expander 5 comprises a level comparator 6 and a variable gain amplifier (gain controller) 8. The video signal from the pedestal clamp circuit 2 is supplied to the level comparator 6 which detects a dark level signal portion below the reference dark level by comparing the level of the video signal supplied thereto with a reference dark level from a reference voltage source 7. The dark level signal portion is supplied to a variable gain amplifier (gain controller) 8, in which the dark level signal portion is lowered and expanded. A gain of the variable gain amplifier 8 is controlled in response to a compared output level from a level comparator 14 which will be described later on.
The dark level signal portion that had been expanded by the variable gain amplifier 8 in the dark detector/dark expander 5 is added to or replaced with the dark level signal portion of the video signal clamped by the pedestal clamp circuit 2 so that a video signal whose dark level portion is expanded is output from an output terminal 13. This output video signal is processed by some suitable video signal processing means (not shown) and supplied to a cathode ray tube (CRT), not shown. Simultaneously, this output video signal is supplied to a dark peak hold circuit 12. The gain of the variable gain amplifier 8 is optimized in response to the addition or replacement of the dark level signal portion.
The dark peak hold circuit 12 detects and holds the dark peak level of the dark level signal portion of the video signal supplied thereto from the adder 4. The dark peak level held by the dark peak hold circuit 12 is supplied to the level comparator 14, in which it is compared with the reference pedestal level of the reference voltage source 3. A compared output from the level comparator 14 is supplied to the gain controller 8 and the gain of the variable gain amplifier 8 is controlled in response to the compared output level of the level comparator 14 such that the dark peak level becomes coincident with the reference pedestal level.
A blanking signal from an input terminal 10 is supplied to the variable gain amplifier 8 and the dark peak hold circuit 12, whereby the gain of the variable gain amplifier 8 is made zero and the dark peak level detection in the dark peak hold circuit 12 is made inactive. Therefore, a last dark peak level in the video signal portion before the blanking period is maintained. Consequently, the signal below the detected dark level in the video signal portion is lowered to the pedestal level and changed to a dark level signal, thereby emphasizing a contrast. Thus, it becomes possible to improve a picture quality from a visual sense standpoint.
Recent high-definition television receivers use a CRT having a face plate with an aspect ratio of 16:9. When a conventional television signal with an aspect ratio of 4:3 is received and reproduced by the high-definition television receiver, the following problems arise.
These problems will be described below with reference to FIGS. 2A through 2D. As shown in FIG. 2A, when a television signal with an aspect ratio of 4: 3 is supplied to and reproduced by a CRT having a face plate with an aspect ratio of 16:9, non-video signal portions (non-picture portions) are produced on both sides of the video signal portion on the picture screen of the CRT. Accordingly, a signal of one horizontal period of that video signal is at the pedestal level in the non-video signal portions as shown in FIG. 2B and becomes lower than the reference dark level (detected dark level).
In this case, as shown in FIG. 2C, when the variable gain amplifier 8 and the dark peak hold circuit 12 are made active within a period T1 including the video signal portion and the non-video signal portions provided at both sides of the video signal portion in which the blanking signal is at low level, the dark peak hold circuit 12 detects and holds the level of the video signal in the non-video signal portions as the dark peak level and the gain of the variable gain amplifier 8 is controlled such that the level of the video signal in the non-video signal portions becomes coincident with the reference pedestal level. As a consequence, the video signal (see FIG. 2D) output from the output terminal 13 is equal to the video signal (see FIG. 2A) from the pedestal clamp circuit 2. Thus, the dark signal portion below the dark detected level of the video signal is not expanded.